1. Field of the Invention
This invention relates to a mechanism for attaching a brake disc to a wheel.
2. Discussion of the Problem
In a disc brake mechanism, a cylindrical metallic disc, which has a pair of smooth, flat, parallel surfaces, is rigidly attached to a wheel. A caliper assembly, which has a pair of disc brake pads, is rigidly mounted on a vehicle frame surrounding a section of the disc such that each pad is adjacent one of the surfaces on the disc. To stop the vehicle, the caliper assembly is actuated, for example by high pressure hydraulic fluid, to clamp the brake pads against the disc surfaces and thereby brake the wheel.
When the brake pads are clamped against the disc a great deal of heat is generated. This heat causes radial thermal expansion of the disc, which moves relative to the wheel, which experiences little temperature change. Since the disc expands and contracts during each braking cycle, the disc mounting mechanism must accommodate this relative movement between the disc and wheel without loosening and without causing undue stress and wear of the elements. Consequently, a direct axial attachment of the disc to the wheel by bolts is not adequate, since disc thermal expansion will exert cyclical bending forces on the bolts and ultimately cause failure through fatigue.
In one of applicants' early designs, elongated holes were provided in the disc to accommodate the cyclical thermal radial expansion and contraction. Each bolt contained a stack of belleville washers adjacent the bolt head and a hardened washer engaging the disc. The belleville washers were provided to maintain adequate bolt torque on the disc despite wear caused by movement of the disc relative to the hardened washer. Upon testing, it was found that the disc did not slide on the hardened washers, but rather the frictional forces between the washer and disc, due to the forces of bolt torque, as maintained by the belleville washers, caused the head of the bolt to move with the disc. Thus, the bolt was bending. It was learned that the cyclical bending was fatiguing the bolts and reducing bolt life to a point significantly below the design life of the disc. The tests also showed that after a number of braking cycles the bolt moved very slightly in and out of the threaded bore in the wheel. This indicated that the threads were wearing. Consequently, this design was unacceptable.
3. Description of the Prior Art
One known method of attaching a disc to a wheel is to provide flexible arms which attach at one end to the disc and at the other end to the wheel. The problem with this design is that it is complex, costly and bulky.
Another known design for attaching a disc to a wheel uses a two-part disc. The disc provides an outer braking ring and an inner fastening ring which is rigidly attached to the wheel. The outer braking ring and the fastening ring are connected by spring loaded, radial pins. In this way, the outer braking ring is isolated from the fastening ring. A problem with such design is that it is quite expensive.
Applicants have devised a mechanism to secure a brake disc to a wheel which accommodates cyclical disc thermal expansion without fatiguing the mounting bolts which is simple and inexpensive.